I have previously begun studies of measles virus polypeptide synthesis utilizing a drug, SV4814, which blocks virus induced cell fusion and cell death, but not other steps in viral replication. The prolonged survival of measles virus infected cells protected from fusion and death by SV4814 made possible labeling and pulse-chase studies of viral proteins, which were analyzed by polyacrylamide slab gel electrophoresis. Evidence for post translational processing of a number of the polypeptides has been found, but the biological significance of these processing mechanisms is unknown. Two viral polypeptides, Fo (fusion protein) and NP (nucleocapside protein) undergo proteolytic cleavage in the infected cell. Phosphorylation has been noted for polypeptides P (nucleocapsid associated protein), NP and possibly M (membrane protein). The M protein probably is dephosphorylated prior to incorporation into mature virions. I propose to first characterize each processing step in acutely infected cells and then to study the step in persistently infected cells in order to explain at a molecular level how measles virus can persist in cells without causing overt fusion and cell death (F protein functions) and with relatively poor production of mature virus by budding (probable M and NP protein functions).